feat: now playing metadata changed hook and event

[tomislav-arambasic]

Summary

Resolves #2158.

Added nowPlayingMetadata hook, getNowPlayingMetadata getter and NowPlayingMetadataChanged event.
Reflect these changes in updateMetadataForTrack (when it's the active track) and clearNowPlayingMetadata for time before deprecation.

useActiveTrack() stays intact, as David suggested in issue, I've done changes as per his recommendations, implementing new event, getter and hook, independently of useActiveTrack.

• Users had no way of obtaining now playing metadata before, for metadata rendering purposes
  • activeTrack event, getter and hook do not provide now playing metadata information - if users would ever change
    metadata of track that is currently playing, there would be no update to it
  • With radio streams, when rendering now playing metadata artist, title and artwork... Radio screen metadata UI can be updated once MetadataTimedReceived event is emitted. But, if user would unmount the screen and then come back to it, now playing metadata is not accessible, until new MetadataTimedReceived event is emitted - when song on the radio stream changes

How it works

• Every time updateNowPlayingMetadata, or updateMetadataForTrack, if it's active track, is called, new metadata is saved into private var inside MusicService / RNTrackPlayer and NowPlayingMetadataChanged event is emitted. clearNowPlayingMetadata reflects these changes accordingly, until it'll be deprecated as announced
• getNowPlayingMetadata returns that variable's value, last stored metadata
• useNowPlayingMetadata fetches that variable's value on mount, saves it into state. Every time NowPlayingMetadataChanged is emitted, it updates the internal state with new metadata. Hook returns metadata
• NowPlayingMetadataChanged is subscribable, event returns metadata object

Testing

• Tested in Example app, using iPhone 13 mini (iOS 17.2) and Google Pixel 3 (Android 12)
• Tested in my app containing podcast RSS feeds and radio streams, using iPhone 13 mini (iOS 17.2) and Google Pixel 3 (Android 12)

Screenshots

Note that artwork gets updated randomly, and because notification metadata and our nowPlayingMetadata update at at different time, they're not the same.

Now playing metadata updated on "Update Notification Metadata Randomly" action		Now playing metadata updated on "Update Current Track Metadata Randomly" action
iOS
Android

Now playing metadata updated for each track via PlaybackService
iOS
Android

Note

@dcvz
I've just noticed that notification metadata is "patched", instead of replaced.
E.g., if we update now playing metadata, or metadata for active track, it'll keep old metadata values for any undefined values in object we're updating with.
I'm not sure if this should be the case, I'd expect users to be able to override notification data completely.
nowPlayingMetadata is replaced completely with new values, even if they're undefined.

[dcvz]

Thanks for taking the time @tomislav-arambasic ! As you saw there's been some conversation on the other thread around options and possibilities.

I'll review this PR later this week!

[dcvz]

Sorry for the delay here! I'm hoping to go through this soon